Reply to comment

I'm not sure your post has an overall point so I will address the items of contention as I see them. I will do this in reverse order since the discussion about carbon isotopes in atmospheric CO2 will be more involved than the first answer.

First, you seem to think I was trying to misrepresent the Nisbet and Weiss paper when I “failed to note” that the author's stated CO2 levels may “...rise dramatically in urban winter rush areas.” The correct quote is “urban winter rush hours,” periods of peak traffic flow. What I stated was that levels could “dramatically increased during rush hour in major cities,” removing the winter time qualification. Far from diminishing the authors' statement, this makes the claim of positive side variability more inclusive and is based on other observations.

Second, you say “atmospheric CO2 produced from fossil fuels has a different isotopic composition and this does provide information on man-caused CO2.” This is true, there are two types of “isotope evidence” obtained by analysis of carbon atoms in carbon dioxide in samples of air: 14C isotope data and 13C/12C isotope ratio data. Atmospheric 14C is constantly replenished by radiation striking the atmosphere. That and its short half-life conspire to make it a poor marker so I will assume that you are referring to 13C.

The ratio of the 13C isotope of carbon to the normal 12C form in atmospheric CO2 has been observed to be decreasing at the same time CO2 has been increasing. The “natural” 13C content of CO2 is just over 1.1%. In contrast, the 13C content of the CO2 produced by burning of fossil fuels is claimed to be slightly smaller – just under 1.1%. The concentration of 12C is increased in biological materials because biochemical reactions discriminate against 13C. Fossil fuels are made from organic material, which has resided in Earth's crust for 100 or more million years, and are partially depleted of 13C. Burning fossil fuels should alter the atmospheric carbon isotope ratio. Hence, some people claim that the change in 13C/12C ratio, usually given as δ13C, serves as a marker for anthropogenic CO2 emissions. There are, however, a number of problems with that hypothesis.

Note that, every year, nature seeps more oil into the ecosystem than humanity spills. This oil is accompanied by methane gas that eventually gets broken down into CO2. The oil is consumed by microorganisms in the oceans and so its carbon also rejoins the short term carbon cycle. There are also a huge number of natural coal seam fires burning around the world, as such fires have since time immemorial. The amount of coal consumed each year has been estimated as being equal to the production of Ohio, Illinois and West Virginia. There are also huge deposits of methane clathrates in deep ocean waters and even in some of the larger freshwater lakes. These also leak gas into the water column and, eventually into the atmosphere. And do not forget volcanoes: the second most prominent gas emission from volcanoes is CO2 (after H2O).

Next, consider that terrestrial plants exchange some where around 240 GT of carbon with the atmosphere each year (according to the IPCC 119 into the atmosphere, 120 absorbed by plants). Some of this comes from decaying old plant matter, including newly thawed tundra, all of which will be partially depleted of 13C. Similarly, the oceans exchange 180 GT and some of the absorption processes are biased with regard to 13C uptake. Human emissions are around 7 GT, and not all of this is from fossil fuels—a lot is from biomass and agricultural activity. The point is, there are a sizable number of natural sources of low 13C CO2 out there that are indistinguishable from anthropogenic sources. Once the CO2 gets “well mixed” into the atmosphere, little can be discerned by taking the δ13C at Mauna Loa.

Speaking of Mauna Loa, there are a number of climate scientists who have analyzed the δ13C data and find no human induced trend at all. Dr. Roy Spencer of the National Space Science and Technology Center at University of Alabama, posted a two part explanation of how detrended 13C data show only natural variability. According to Dr Spencer, “The long-term increases in carbon dioxide concentration that have been observed at Mauna Loa since 1958 could be driven more than by the ocean than by mankind’s burning of fossil fuels.” See part 1 and part 2 of his article for details. His own website has a further analysis here.

I personally have no doubt that humans are contributing CO2 to the atmosphere, but the carbon cycle is so complex, the exchange amounts so large and the natural sources so varied using 13C as an anthropogenic marker is a fools game. But beyond that, looking for an anthropogenic marker in the well mixed atmosphere misses the point of the article. Nisbet and Weiss were pointing out that, unless you know the exact sources of human emissions on a local level, it is impossible to regulate those emissions or measure the effectiveness of such regulation. That is why they called for a worldwide network of ground, air and satellite sensors to pinpoint the actual sources of CO2 emissions. You obviously skimmed the article but you did not understand it.